1. Field of the Invention
This invention relates to optical recording media, each, having a recording layer containing at least a dye and a reflective layer, both on a transparent substrate, and permitting recording and reproduction with a laser beam of a wavelength selected from 620 to 690 nm, and also to compact disks (hereinafter called "CD-Rs") permitting recording and reproduction with a laser beam of a wavelength selected from 770 to 830 nm.
2. Description of the Related Art
Optical recording media of the single plate type that a dye is arranged as a recording layer, a metallic reflective layer is applied over the recording layer to provide a greater reflectance and a protective layer is arranged over the reflective layer are disclosed, for example, in Optical Data Storage, 1989, Technical Digest Series, Vol. 1, 45 (1989). Further, media making use of a cyanine dye or a phthalocyanine dye, the latter dye being useful in the present invention, in the recording layers of such media have been put as CD-Rs on the market.
These CD-R media are based on the Orange Book Standards, and have a reflectance of at least 65% to a light of wavelength (.lambda.1) selected from 770 to 830 nm and absorb light of this .lambda.1. They are hence characterized in that they permit recording with a semiconductor laser of 780 nm and also reproduction by a commercially-available CD player or CD-ROM player with a semiconductor laser of 780 nm mounted thereon.
On the other hand, current optical recording media have a capacity as small as 650 MB and, when information of large capacity such as digital movies are recorded, the recordable time length is as short as 15 minutes or less. Further, under the current tendency toward smaller equipments, the conventional recording density will lead to the problem of insufficient capacity if media are rendered smaller.
Recently, semiconductor lasers of a wavelength (.lambda.2) selected from 620 to 690 nm have been developed, thereby progressively making it possible to perform high-density recording and/or reproduction. Use of these semiconductor lasers are now under study for the development of high-density recording media having a recording capacity 5 to 8 times as much as the conventional media and also of players which are compatible with such high-density recording media and can hence play them. In particular, a system which permits digital recording of movies for 2 hours or longer has now been developed as a DVD (digital video disk) system, and marketing of its play-only media and players is about to be started soon.
As one of such high-density recording media, there can be mentioned a play-only medium which has been fabricated, like conventional CDs and CD-ROM media, by forming pits upon formation of a substrate and arranging a reflective layer of aluminum. Because this play-only high-density recording medium has a reflectance as high as 70% or even greater, high-density compatible players for such high-density recording media are designed to permit playing play-only media having a high reflectance of 70% or greater. Needless to say, it is also desired for these players to permit playing conventional media, for example, CDs, CD-ROMs and CD-Rs.
Conventional CD and CD-ROM media are for play only and their fabrication process is the same as that employed for the above-described play-only high-density recording media. They have a high reflectance of 70% or greater to a .lambda.2 laser beam. They can therefore be readily played even by the high-density compatible players.
On the other hand, the CD-R media which are currently furnished to the market certainly have a reflectance of 65% or higher to laser beams having a wavelength of about 780 nm and can therefore be played even by commercially-available CD or CD-ROM players. They however develop numerous drawbacks when player with a .lambda.2 laser beam. The reflectance becomes very small, that is, as low as 10% or less. Moreover, the degree of modulation is also small and, on top of that, the reflectance of a recorded area becomes greater than that of an unrecorded area, in other words, so-called "low to high" recording takes place. This record is opposite in polarity to those of ordinary CDs ("high to low" records) and is not preferred. In addition, substantial distortion is observed in the recorded waveforms. Due to these numerous drawbacks, it has heretofore been difficult to play conventional CD-R media by a high-density compatible player with a laser of a wavelength (.lambda.2) selected from 620 to 690 nm.
For example, U.S. Pat. No. 5,090,009 discloses a CD-R medium fabricated by successively stacking a dye-containing recording layer, a reflective layer and a protective layer on a substrate and also a medium provided with an interference layer between the substrate and the recording layer or the recording layer and the reflective layer of the CD-R medium. Also disclosed therein are optical constants of the recording layer and its thickness, which are required to meet the CD Specification (the Red Book) and to permit recording. It is certainly true that the disclosed media absorb a portion of a 780 nm laser beam employed for ordinary CDs and provide a reflectance of 70% or higher, thereby permitting recording and reproduction with a laser beam of this wavelength. They are however not intended to perform recording and/or reproduction with a laser beam of a wavelength (.lambda.2) selected from 620 to 690 nm, and nothing is disclosed about the reflectance to the laser beam of the above wavelength, the optical constants of the recording layer and the optical pass length of the interference layer. The above patent discloses many examples of media without any interference layer and also of media with an interference layer composed of an inorganic compound or a polymer. However, the medium making use of an indodicarbocyanine dye in its recording layer has a reflectance as low as 10% or even less to a laser beam (.lambda.2) of a wavelength selected from 620 to 690 nm as will be described subsequently in a comparative example, and recording thereon is the "low-to-high" recording that the reflectance of a recorded area is higher than that of an unrecorded area. This medium is therefore not preferred. The lower reflectance at 620 to 690 nm can be attributed to the fact that optical characteristics of dyes employed in CD-R media have high wavelength-dependency, the above-described indodicarbocyanine dye has a very large absorption at 600 to 750 nm when measured in the from of a recording layer, and the thickness and optical constants (refractive index, extinction coefficient) of each CD-R medium are designed to have a large reflectance to a light beam around 780 nm. Further, the above patent also discloses, in Examples 8 and 14, media each of which is provided with a recording layer of a t-butyl-substituted phthalocyanine dye different from a phthalocyanine dye employed in a recording layer in the present invention and with an interference layer composed of a polymer or an inorganic compound. Although these media successfully meet the CD Specifications insofar as their reflectances and modulation degrees are concerned, substantial distortion is observed in the waveform of reproduced signals when signals recorded at the same pulse width modulation as that employed for CDs are reproduced with a laser beam of a wavelength (.lambda.2) selected from 620 to 690 nm. Due to a large error rate and jitter, such media cannot be played by any high-density compatible player.
European Patent No. 19329 discloses a medium, which makes use of a VO-phthalocyanine dye as a recording layer and is provided, over the recording layer, with an 300 nm interference layer made of cellulose. However, this patent is also intended to improve the reflectance and recording sensitivity to a particular single wavelength. Its recording layer is hence not optimized so that, as in the present invention, recording and reproduction can be performed with two laser beams, one being a laser beam of a wavelength (.lambda.1) selected from 770 to 830 nm and the other being a laser beam of a wavelength (.lambda.2) selected from 620 to 690 nm. When signals recorded on the above medium at the same pulse width modulation as that employed for CDs are reproduced with the laser beam of the wavelength (.lambda.2) selected from 620 to 690 nm, substantial distortion is also observed in the waveform of the signals so reproduced. Again, due to a large error rate and jitter, the above medium cannot be played by any high-density compatible player.
U.S. Pat. No. 5,124,067 discloses many dyes similar to those useful in the recording layer of each medium according to the present invention and also many media making use of such dyes. The media disclosed, for example, in Examples 98, 100 and 102 of this patent can meet the CD Specification but, when played with a laser beam of a wavelength (.lambda.2) selected from 620 to 690 nm, the recording mode becomes "low to high", the reflectance is as low as 10% or less, large distortion is observed in the reproduced waveform of signals recorded at the same pulse width modulation as that employed for CDs, and the error rate and jitter are significant. They cannot accordingly be played by any high-density compatible player.
Japanese Patent Laid-open No. 281287/1991 discloses a medium making use of a mixed dye as a recording layer, said mixed dye consisting (A) a dye having an absorption curve with a longer-wavelength-side end located at 780 nm and a complex refractive index characterized by a value of 0.2 or smaller as the absolute value of an imaginary part of its refractive index as measured at 780 nm and (B) a trimethine cyanine dye having an absorption maximum on aside of shorter wavelengths than the absorption maximum wavelength of the dye (A). This medium is excellent in durability and meets the CD Specification. It is however not optimized that reproduction can be performed with a laser beam of a wavelength (.lambda.2) selected from 620 to 690 nm. As is understood from its Examples, the trimethine cyanine dye and the pentamethine cyanine dye are used in proportions of 50 wt. %, respectively, so that the absorption for the laser bean of the wavelength (.lambda.2) selected from 620 to 690 nm is excessive, resulting in a reflectance lower than 15%. The above medium can therefore hardly be played by any high-density compatible player.
Japanese Patent Laid-open No. 336086/1994 discloses a medium making use of a mixture as a recording layer, said mixture consisting of trimethine and pentamethine cyanine dyes of particular structures. This medium is intended to permit recording and reproduction with laser beams of 780 nm and 488 nm. The ratio of the trimethine cyanine dye to the pentamethine cyanine dye is 1:10. When the pentamethine cyanine dye is used in such a large proportion, the absorption for a laser beam of a wavelength (.lambda.2) selected from 620 to 690 nm is excessive, resulting in a reflectance lower than 15%. The above medium can therefore hardly be played by any high-density compatible player.
Japanese Patent Laid-open No. 40162/1994 discloses a medium making use of a trimethine cyanine dye in its recording layer. This medium is intended to permit recording and reproduction with a laser beam of 630 nm and, as the recording layer does not show any absorption for a laser beam of 780 nm, does not permit recording with the laser beam of 780 nm.
Japanese Patent Laid-open No. 290835/1991 discloses a medium provided with an interference layer of a low molecular organic substance between a recording layer and a reflective layer made of an aluminum alloy. This medium is provided with the interference layer to achieve a reflectance of 70% or higher at 780 nm by using the aluminum alloy instead of costly gold in the reflective layer. Although it can certainly provide a reflectance as high as 70% or even greater at 780 nm, the reflectance to a light beam of a wavelength (.lambda.2) selected from 620 to 690 nm is low so that the above medium cannot be played by any high-density compatible player.
As has been described above, there is no optical recording medium, which has a recording layer containing at least a dye and a reflective layer, provides a reflectance of 65% or higher to a laser beam of a wavelength (.lambda.1) selected from 770 to 830 nm, has high sensitivity and excellent recording characteristics, can be played by a commercially-available CD or CD-ROM player, and can also be played even by a player equipped with a laser of a wavelength (.lambda.2) selected from 620 to 690 nm and compatible with high-density recording media.